MUSEUM OF COMPARATIVE ZOOLOGY. 543 



amount of nuclear fluid '? Its increase in size appears to be intimately- 

 connected with its migration, and the migration is apparently in response 

 to the direct attractive influence of the female pronucleus, with which it 

 is ultimately fused. In cases where the latter has attained its full size, 

 its power of attraction will be greater, as its mass is greater, than at 

 any earlier period in its growth, and the migration of the male pro- 

 nucleus will be correspondingly more rapid, so that it will have less 

 time to incorporate wath itself substances from the protoplasm. It will 

 reach the female pronucleus before it has acquired its normal size; but 

 this it may subsequently attain by continuing to grow after encountering 

 the latter, as Selenka has shown to be the case in Toxopneustes. This 

 may perhaps explain the experimental cases, as well as that of Toxo- 

 pneustes, where the female pronucleus takes a position in the centre of 

 the yolk. It cannot, however, be claimed that migration and growth 

 stand in the relation of cause and effect, since in certain cases (eccentric 

 female pronucleus), when the male pronucleus arises near the animal 

 pole, the extent of its migration is more limited than when it makes its 

 appearance near the opposite pole ; and yet it attains in both cases the 

 same dimensions, and probably grows with the same rapidity. It can 

 be said that both migration and growth appear to depend on the exist- 

 ence of certain conditions which are established with the elimination of 

 the substance of the polar globules, but not that those conditions are 

 fulfilled by the liberation of nuclear fluid at the time of the conversion 

 of the germinative vesicle into an amphiaster ; for the observations of 

 Whitman on the "quiescent state" of the Qg^ in Clepsine seem to aff'ord 

 the most satisfactory proof that the metamorphosis of the vesicle may 

 transpire long before the enlargement of the male pronucleus, and 

 nearly all observers concur in the statement that the two pronuclei 

 arise at nearly the same time in cases where spermatization has preceded 

 maturation,* From this it appears to me that the growth of the male 

 aster, instead of corroborating, offers serious obstacles to the acceptance 

 of Hertwig's explanation. 



* From the evident dependence of the migration and growth of the male pro- 

 nucleus on the existence of a more or less developed female pronucleus, it appears 

 reasonable that with the detachment of the polar cells the character of the nuclear 

 substance is so far altered that it exerts on the male pronucleus an attraction of 

 which it was incapable when still joined to the nuclear substance that is removed 

 with the polar cells. Whether this implies a greater difference than exists between 

 the halves of the nuclear plate in ordinary cases of cell division, may be questioned. 

 It should at least be remembered that there are in all cases indications of a mutual 

 repulsion between the lateral zones of fibre thickenings. 



